Only certain battery voltages exist, because this voltage depends intrinscally on the battery's chemical composition.

learn.sparkfun.com/tutorials/battery-technologies/all (CC BY-SA) has a very good summary list, reordered from lowest to highest voltage:

Battery Shape	Chemistry	Nominal Voltage	Rechargeable?
AA, AAA, C, D (Rechargeable)	NiMH or NiCd	1.2V	Yes
AA, AAA, C, and D	Alkaline or Zinc-carbon	1.5V	No
Coin Cell	Lithium	3V	No
Silver Flat Pack	Lithium Polymer (LiPo)	3.7V	Yes
9V	Alkaline or Zinc-carbon	9V	No
Car Battery	Six-cell lead acid	12.6V	Yes